Dielectric method for the determination of aw

The measurement of a specific surface area with water vapour by the classical methods of sorption always proceeds by the conditioning of the material under constant P/P0 of the given vapour. This makes the method very slow, typically lasting a few days. Dielectric spectroscopy in the microwave domain, in which the relaxation of free water is strongly discriminated from that of bound water, allows the quantification of the adsorption and desorption of water vapour on the surface of solid materials. In this paper we present a new dielectric methodology, using a special resonant cavity, for simultaneously obtaining the dielectric parameters and the weight losses of hydrated material under test. A theoretical approach was developed to calculate (during desorption or adsorption), the value of water activity aw. For that, a resonant cavity was projected and tested to allow calculation of the specific surface area with water vapour of a paper sheet, using the BET model.